The British mission in Afghanistan will continue despite the loss of six soldiers who were caught in a huge explosion that destroyed their armoured vehicle, defence officials have said.

If, as expected, the men are confirmed to have been killed in the blast, it will take the death toll of British personnel killed in Afghanistan to 404. It will also represent the biggest single loss of UK life in a single incident in Afghanistan since 2006, when an ageing RAF Nimrod crashed in the country shortly after mid-air refuelling, with the loss of all 14 people aboard.

Passing the symbolic milestone of 400 deaths would refocus attention on the reason for the presence of British troops in Afghanistan, the timetable for their removal and the capability of the Afghan army and police force to keep the country secure once coalition troops have left.

Defence officials said the deaths would have no effect on strategy or tactics deployed by British troops. If anything, it would encourage military commanders to stay the course, they said. However, officials indicated ministers were well aware of war weariness among the public.

An investigation is under way to find out what caused the explosion that wrecked the Warrior vehicle as it patrolled the border of Helmand and Kandahar in southern Afghanistan on Tuesday. The force of the blast led to early speculation that the vehicle was hit by an old mine, possibly from the Soviet era, but an initial examination of the Warrior suggested it was probably a large Taliban roadside bomb.

The bodies of the soldiers were still being extracted from the wreckage last night in an operation hampered by bad weather, defence officials said. The soldiers are expected to be named on Thursday.

David Cameron described the loss of the men – who were described by the Ministry of Defence as missing, believed killed – as a "desperately sad day for our country". He said it was a "reminder of the huge price that we are paying for the work we are doing in Afghanistan and the sacrifice that our troops have made and continue to make".

But the prime minister pointed out that the men were carrying out important work to prevent Afghanistan becoming a haven for al-Qaida to plan attacks on the UK or its allies.

Five of the soldiers presumed dead were from the 3rd Battalion the Yorkshire Regiment and one from the 1st Battalion the Duke of Lancaster's Regiment.

At Battlesbury barracks in Warminster, Wiltshire, where the 3rd Battalion the Yorkshire Regiment is based, the news was broken to families and friends of the missing men.

Service personnel past and present, army families and townsfolk arrived at the gates to lay flowers and express their sadness.

Stacey McLaughlin, 21, whose husband, John, is to fly to Afghanistan in April laid flowers with their daughter, three-year-old Olivia-Mai. Speaking through tears, she said: "They are true heroes. This loss will be felt badly by all the soldiers out there and the families back home. I just hope John comes back home safely when he goes out there."

Hayley Dymek, 22, whose husband, Private Darren Dymek, 24, is in Afghanistan with the Yorkshire regiment, said: "I panicked this morning when I first heard that six soldiers had been killed. But then I found out that the families had been informed, so I knew Darren was safe. This really brings home the dangers of what they are doing out there."

Downing Street said last month that British troops would end their lead combat role in Afghanistan by the end of next year. This will lead to Afghan security forces being in sole charge of combat operations against insurgents by the end of 2014, when most of Britain's 9,500-strong contingent will have returned home. The timeframe may well be shortened at the Nato summit in Chicago in May.

However, Nato commanders in Afghanistan have told Downing Street and the White House that they want to keep levels of International Security Assistance Force (Isaf) as high as possible between now and the end of the "fighting season" next year – September 2013.

They insist this is the best way of ensuring the country does not descend into chaos before the final withdrawal of combat troops. If Cameron agreed to Nato's proposal, about 9,000 UK troops would remain in Helmand until autumn next year.

They would be under more pressure too. Sixteen thousand US Marines are withdrawing from Helmand within months, and although Philip Hammond, the defence secretary, has said Britain would not "backfill", Afghan forces are unlikely to be able to shoulder the burden of security alone.

Privately, commanders also warn the Isaf coalition could disintegrate if either country signals an early exit. "That is the biggest danger for Afghanista," said one. "A hasty and chaotic exit must be avoided." But political pressure is mounting on both sides of the Atlantic to accelerate the drawdown.

The loss of the British soldiers this week is bound to raise questions about their resources. Cameron accepted there was a need to make sure personnel had all the equipment they needed to keep them "as safe as possible".

But the MoD was quick to quash suggestions that the vulnerability of the Warrior could be blamed for the deaths. Warrior had been upgraded throughout its service in Iraq and Afghanistan to try to combat the threat from roadside bombs and mines. A £1bn upgrade of the vehicle was announced at the end of last year to extend the life of the vehicle to 2040 and beyond but the MoD stressed this was not in response to "any identified deficiencies" in its armour.

A spokesman said: "Warrior provides some of the highest levels of protection available but sadly no armoured vehicle can provide absolute protection from the very largest explosions."

The chief of the defence staff, General Sir David Richards, said the men were doing "a dangerous but important job", adding: "Increasingly the Afghans themselves are taking the lead in providing security across Helmand. This transition is allowing Afghans to gain the confidence to reject the Taliban and live normal lives."

Some experts remain unconvinced. Gareth Price, senior research fellow on the Asia programme at Chatham House, suggested the explosion could have been designed to send a message to the Afghan people that the national army could not guarantee their safety.

Anthony Glees, director of the centre for security and intelligence studies at the University of Buckingham, said: "The fact that these people were killed by an IED (improvised explosive device) might suggest not just that this is a very dangerous place but that the Afghans aren't particularly good at delivering security."